Why faces have a dark texture? Already recalculated normals
Since you can't upload your model, I will try to point out the problems with your plane based on your screenshots. As stated in the comments, your issue is caused by flipped normals. You also have overlaying geometry, that can be caused by


duplication of the object at the exact same spot of the original mesh 
duplication of all faces "inside" (in Editmode) of the objects


One of the objects has normals facing inwards, the other object faces outward. That explains why the normals are consistent, but look inconsistent. Find the Object or faces that point inwards (dark blue) and delete them or move them to another layer.

There were also some additional issues with your model, they are not part of the question but I want to point them out anyways.




faulty mirror modifier: (sorry for the typo in the screenshot)
the mirror axis is off, you have intersecting geometry on your Y-axis  
there are several places with poor topology. I would suggest you to remodel parts of the object with cleaner topology. I hope you didn't pay too much for this model, its quality isn't very good.


if you want me to take a closer look, send the file to ian.smith[ätt]posteo[dott]de